Resistor



1953 L. H. BERKELHAMER 2,628,300

RESISTOR Filed Dec. 18, 1948 IN V EN TOR.

wL-si Cy Patented Feb. 10, 1953 RESISTOR Louis H. Berkelhamer, Chicago, Ill., assignor to David T. Siege], Glencoe, 111.

Application December 18, 1948, Serial No. 65,990

8 Claims.

This invention relates to electrical circuit elements, and particularly to wire coil circuit elements such as resistors and to methods for effecting the manufacture thereof.

Certain electrical circuit installations, of which a resistor installation may be taken as exemplary, require that the resistor be subjected to a high degree of thermal shock, viz., extreme and rapid temperature changes. Wire coil resistors which are coated with conventional varnishes or lacquers are unsuitable for such installations due to the fact that the varnishe and lacquers do not withstand high temperatures Without deterioration and failure in their insulating properties. Wire coil resistors which are coated with vitreous enamel are adapted for high temperature installations inasmuch as vitreous enamel has high thermal resistivity and maintains an effective insulating coating for the coil even under condition of continuous high temperature. However, in installations wherein the resistor is subjected to thermal shock, that is, extreme and rapid temperature changes, the vitreous enamel exhibits a tendency to craze, and the multiplicity of surface cracks thus formed even though relatively minute destroy the effectiveness of the enamel coating particularly in respect to moisture resistivity, durability, and other operating conditions.

In accordance with the present invention a wire coil resistor structure or the like is provided which utilizes the advantages of vitreous enamel as a thermal protection for the wire coil or other electric circuit component, while at the same time providing effective means for protecting the vitreous enamel and the coil from the effects of thermal shock.

Accordingly it is an object of the present invention to provide an improved wire coil electrical resistive element or other circuit component, and specifically a resistor, which will effectively resist thermal shock as well as continuous operating conditions of hi h temperature.

More specifically stated, it is an object of the invention to provide an improved structure as defined, which employs and utilizes the thermal resistivity of vitreous enamel as a protective coating for the wire coil or other electrical element, and additionally provides effectiv protective means for the vitreous enamel coating in respect to thermal shock; whereby to provide an improved structure which may be effectively utilized under operating conditions of high temperature, and extreme thermal shock, as well as under normal and conventional conditions of operation.

A further object of the invention is to provide an electrical element and specifically a wire coil electrical circuit element, or resistor, wherein silicone is employed in an improved manner, as a protective coating for the wire coil and/or other element parts.

In accordance with the present invention the circuit element, or resistor particularly disclosed, is provided with a ceramic core, a resistive wire coil mounted thereon, a vitreous enamel coating for the coil, and coating means including silicone for the vitreous enamel, fabricated and applied in a manner as will hereinafter be more specifically pointed out.

Various attendant objects, feature and advantages of the invention will be apparent from the following specification, when taken in connection with the accompanying drawings, wherein a preferred embodiment of the invention is set forth for purposes of illustration.

In the drawings, wherein like reference numerals refer to like parts throughout:

Fig. l is a perspective view of a resistor element constructed in accordance with and em bodying the principles of the invention, in accordance with one preferred embodiment thereof;

Fig. 2 is an enlarged partial longitudinal sectional view of the resistor structure of Fig. 1 taken as indicated by the line 2-2 thereof;

Fig. 3 is a further enlarged and detail illustrative view showing the manner in which the wire coil is embedded in the vitreous enamel and covered thereby, prior to the application of the silicone coatings;

Fig. 4 is a general perspective view of the resistor, prior to the application of the silicone coatings; and

Figs. 5, 6 and 7 are views illustrating the manner in which the silicone coatings may be applied to the resistor unit, and the curing of the coatings effected.

In the drawings, the invention has been illustrated as applied to an electrical resistor of the wire coil type, to which it has particular application. It is to be understood, however, that the invention is also adapted for use with other electrical circuit units or elements containing circuit components to be protected, and particularly wherein high temperature operating conditions and conditions of thermal shock are to be encountered.

Referring more specifically to the drawings, in Fig. 1 there is illustrated a resistor comprising a I body 10 carrying at its opposite ends a pair of terminal lugs as indicated at l2 and [4. These terminal lugs have projecting tine portions for connection to electric circuit elements and band portion engaging the body of the resistor, the resistor wire coil being bonded at its opposite ends to the lugs for electrical interconnection therewith in the usual manner.

The details of the resistor body are more particularlyillustrated in Fig. 2 wherein it will be seen that the body comprises a ceramic core in the form of a cylinder I6 upon which is wound the resistor wire IS with its convolutions in predetermined spaced relation. The Wire convolutions are embedded within and covered by a coating 20 of vitreous enamel, which coating is in turn encased within a pair of protective coatings22 and 24 containing silicone, and-the details of which will be presently described. The manner in which the convolutions of the resistance Wire are maintained in spaced relation, and are embeddedand encased within the vitreous enamel coating 20 is more particularly illustrated in Fig. 3, wherein it'will'b seen that each wire convolution is individually embedded into the vitreous "enamel layer. In Fig. 4 the details of construction are further shown, the right end of the 'resistor'being shown with the terminal lug mountedupon the ceramic core i6, and the wire coil being electrically bonded thereto as indicat'edat 26 and the'left} end of the resistor being shown with thefvitrecus enamel layer or coating 26 'applied'to and embracing the wire coil convolu- 'tions. 7 g r Silicones are a class of organo-silicon oxide polymers'having insulating properties and which canbe cured by the action of heat. In fabricatin'gthe silicone layer or coating 22 the silicone such as'for example silicone liquid is mixed with a thinning agent such as carbon tetrachloride or naphtha, the mix being stirred or beaten to form a creamsuspension of suitable viscosity. In the particular embodiment herein described, the resistor comprising the ceramic core, the wire coil andterminm lugs, and the'vitreous enamel layer 20 is immersed into a body 28 of the suspension thus formed, as indicated in Fig. 5, the liquid level within the container tank 3i! being maintained in respect to the depth of immersion so that the proje'ctingends of the terminal lugs remain above the liquid'bodyand thus remain uncoated. After application of the coating the resistor is placed within a heating device as indicated in Fig. '7, "such as an electric oven 32, and subjected to suf- =ficientheat=to effect a curing of the silicone coating applied. The curing temperature'rnay vary depending upon the length of time of the heat application, 'the'temperature however being maintained well below the firing temperature of vitreous enamel. The coating 22 may, if desired,

up by dipping operations it will be understood that the coating may be applied by various means such" as pouring, spraying, or otherwise. It will further be understood that the vitreous enamel layer or coatingiil may be applied to the resistor, and fired, 'in any approved manner by details forming no part of the present invention.

After the formation of the coating 22, the outer silicone coating tZ l is applied as an outside pro- 'te'cti ve layer. In formingthe coating 24 silicone liquid, with a thinrler such as' toluene or'naphtha 4 is mixed with a powdered mineral filler, preferably silica, the mix being stirred and beaten to make a suspension of the desired consistency or viscosity for further dipping operations. The resistor is dipped into the suspension mix thus formed, as indicated at 3 5 in Fig. 6, in a manner similar to the forming of the coating 22 as previously described in reference to Fig. 5; and the formed coating 24 is then heat cured as previously described in reference to Fig. 7. The coating 24 may be formed as a single layer or by a series of successive dipping and heat curing operations, or

by a succession of dipping operations and a single 'heatcuring' operation, as desired.

It has been found that an electrical element --thusly-constructed is particularly resistant to thermal shock. The vitreous enamel layer 20, which is highly heat resistant, forms a firm bond and protective coating for the convolutions of the wire. The silicone layer 22 providesa'highly efilcie'nt and effective thermal insulation for the' 'vitreous enamel, at least to the extent that rapid and violent temperature changes are prevented from crazing or otherwise disintegrating the body or surface of the vitreous enamel layer. The outer layer 24 which comprises a cured suspension mix ofliquid silicone and a powdered mineral filler provides increased mechanical strength and'resistance to surfaceab'rasion as compared with the silicone layer'2i2,while at the same time combining with and augmenting the action of said layer 22 in providing an effective thermal protection for the vitreous enamel and its ernbeddedwire coil. The silicone layer 22 forms a firm bond between the vitreous enamel and the composite layer 240i silicone and the mineral filler, wherebyto provide a mechanically strong unit sufiiciently resistant to surface abrasion and highly'resistant to high temperatures and thermal shock. The ua1- ity of the vitreous enamel, after the original firing thereof, is in no way-impaired by the lower temperature heat curing operations applied in" the formation of the silicone layers 22 and 24.

It is obvious that various changes may be made in the specific structures and method steps heretofore set forth and described without departing from the spirit of the invention. 'The'invention' is accordingly not to be limited, except as indicated in the following claims.

The invention is hereby claimedas follows:

1. An electrical resistor comprising a base, a resistance element carried by the base, a coating of fired vitreous enamel overlying said resistance element and providing insulation thereforpan'd an insulation coating body containing heat cured silicone overlying said vitreous enamel.

V 2. An electrical resistor 'comprisin'g a base, a resistance element carried by the base,a coating of vitreous enamel overlyingsaid resistance element and providing insulation therefor, and a pair of insulation coatings overlying the vitreous enamel, one of said pair of coatings comprising silicone and the other comprising silicone and a powdered mineral filler.

3. A resistor comprising a ceramic core, acoil of resistive wireencompassing the core, a sleeve of vitreous enamel overlying the wire andwithin which the wire is embedded, a sleeve of insulating material containing silicone overlying 1 and embracing the vitreous enamel sleeve, and a sleeve of insulating'material comprising-silicone and a powdered mineral filleroverlying'the first named silicone sleeve.

4. The method of makingan electrical resistor comprising mounting a resistance element -upcn a base, applying a coating of vitreous enamel over the resistance element, and thereafter applying a coating containing silicone and a powdered mineral filler over said first named coating.

5. The method of making an electrical resistor comprising mounting a resistance element upon a base, applying a coating of vitreous enamel over the resistance element, firing said enamel, thereafter applying a silicone containing coating over the fired vitreous enamel, and heat curing said last named coating at a temperature materially less than the firing temperature for the enamel.

6. The method of making an electrical resistor comprising mounting a resistance element upon a base, applying a coating of vitreous enamel over the resistance element, firing the vitreous enamel, thereafter applying a pair of protective coatings over the vitreous enamel, one of said pair of coatings comprising silicone and the other comprising silicone and a powdered mineral filler, and heat curing said pair of protective coatings.

7. The method of making a resistor comprising winding a resistive wire upon a ceramic core, applying terminal lugs to the core and electrically connecting the wire thereto, embedding the containing silicone and a powdered mineral filler r over the first named silicone coating, and heat LOUIS H. BERKELHAMER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,258,220 Rochow Oct. '7, 1941 2,398,686 Zander Apr. 16, 1946 2,425,032 Deyrup Aug. 5, 1947 2,460,795 Warrick Feb. 1, 1949 2,473,887 Jenning et a1 June 21, 1949 2,491,965 Ganci Dec. 20, 1949 OTHER REFERENCES Marbaker, Coatings for Wire-Wound Resistors, December 1945, pages 329-342 of Journal of the American Ceramic Society, (pages 336-337 relied upon). 

1. AN ELECTRICAL RESISTOR COMPRISING A BASE, A RESISTANCE ELEMENT CARRIED BY THE BASE, A COATING OF FIRED VITREOUS ENAMEL OVERLYING SAID RESISTANCE ELEMENT AND PROVIDING INSULATION THEREFOR, AND AN INSULATION COATING BODY CONTAINING HEAT CURED SILICONE OVERLYING SAID VITREOUS ENAMEL. 